1. Field of the Invention
The present invention relates to the power train of a motor vehicle. More specifically, the present invention relates to a power transfer unit in the power train for distributing power to the front and rear wheels of the vehicle.
2. Description of the Prior Art
In the past, most automobiles in the United States utilized a rear wheel drive power delivery scheme. In adapting these rear wheel drive schemes into four wheel drive applications, a transfer case was, and often still is, positioned at the output of the transmission assembly. When engaged, the transfer case diverts a portion of the power coming from the transmission assembly from the rear wheels to the front wheels.
Currently in the United States, a significant portion (if not a majority) of new automobiles are front wheel drive based vehicles. In a front wheel drive vehicle, both the engine and the transmission assembly are typically transversely oriented in the vehicle. By positioning the power plant and transmission assembly transversely in the vehicle, more direct coupling of the transmission assembly to the vehicle""s transaxle and front wheels can be achieved. In doing so, a front wheel differential is incorporated directly into the transmission assembly itself.
With front wheel drive vehicles themselves becoming a mature market, a recent trend in the automobile industry has been to adapt these front wheel drive schemes for all or four wheel drive applications. This is accomplished by providing a power transfer unit that diverts a portion of the power from the front wheels to a rear wheel drive shaft and subsequently the rear wheels.
Seen in FIGS. 1a and 1b is a typical prior art power transfer unit 10. Such a power transfer unit 10 couples to the transversely oriented output of the transmission assembly and includes a housing 12 within which is located a gear set 14 comprised of a parallel gear set 16 and a non-parallel gear set 18.
The parallel gear set 16 includes a cylindrical extension 20 that operates as its input and is coupled to the output 22 of the transmission assembly by way of a splining engagement 24. The cylindrical extension 20 itself extends off of a first gear wheel 26 or may be a sleeve to which the gear wheel 26 mounts.
From the first gear wheel 26, power is transferred through a second and third gear wheel, respectively 32 and 34, each supported on bearings 36 for rotation about axes 38 and 40 parallel to the rotational axis 28 of the first gear wheel 26.
The non-parallel gear set 18 includes a bevel ring gear 44 that is mounted to a shaft or sleeve 42 onto which the third wheel gear 34 is also mounted. The bevel ring gear 44 engages a bevel gear 46 mounted to another shaft 48 whose axis is generally perpendicular (and therefore non-parallel) to that of shaft 42. Mounted to an opposing end of the shaft 48 is an output member 50 that includes a flange 52 and appropriately located bolt openings 54. The latter features enable the output member 50 to be bolted to a rear drive shaft (not shown).
As seen in FIG. 1a, the axis 56 along which the parallel gear set 16 engages with one another corresponds with the axis about which the output member 50 rotates. As a result of the locating of the power transfer unit 10 relative of to the output of transmission 22, it is clear that this axis 56 may be offset from the centerline of the vehicle. As seen in FIG. 1b, the axis 28 of input into the power transfer unit 10, is vertically or elevationally offset relative to the output axis 58 about which the output member 50 rotates. This xe2x80x9cdropxe2x80x9d or height decrease results from the relative positioning of the first, second and third helical gears 26, 32 and 34 of the parallel gear set 16. The non-parallel gear set 18 is a hypoid beveled gear set in that the axis of rotation 40 of the bevel ring gear 44 does not intersect the axis 58 of rotation of the bevel gear 46.
While prior power transfer units work sufficiently well for-their intended purposes, their construction limits the extent to which their size can be reduced (thereby making the engine bay unavailable for other components). This also limits packaging flexibility of the units themselves. Normally front differentials in transaxles are of the bevel type and require a large amount of axial space. As the transaxle package grows axially, it interferes with having equal length halfshafts and furthermore steering, suspension, and engine mount/roll restrictor placement. Additionally, current power transfer units utilize a series of helical gears, spur gears or a chain gear to initially receive power from the transmission assembly. While not seen in FIGS. 1A and 1B, power may then be provided to a bevel gear set and subsequently to the rear wheel drive shaft. This scheme applies a significant amount of torque to helical (or other type) gears and also imparts significant bending forces into the drive train. The latter drawback occurs because of the distance between the bevel gear set""s transverse axis and the output axis of the transmission assembly.
It is therefore an object of the present invention to overcome the above and other limitations of the prior art.
It is also an object of this invention to reduce the bending forces applied to the drive train.
A further object of this invention is to increase the available packaging space in the motor vehicle""s engine bay.
Another object of this invention is to provide a power transfer unit having the front differential incorporated therein.
The present invention overcomes the limitations of the prior art by integrally packaging the primary components of a power transfer unit, a non-parallel gear set, a parallel gear set, and input and output members within a common housing. With the present invention, the non-parallel gear set is located to first receive, in conjunction with input of the power transfer unit, power being transmitted from the output of the transmission assembly.
The output of the transmission assembly is coupled to the input member and to a first bevel gear ring. The first bevel gear engages a second bevel gear, such as a hypoid pinion gear, mounted to or formed with a shaft; this shaft being oriented generally perpendicularly to the rotational axis of the hypoid ring gear.
On the opposing end of the shaft is a first gear wheel, the drive sprocket wheel, in the parallel gear set of the power transfer unit. The drive sprocket wheel transfers rotation to a second gear or sprocket wheel through a transfer chain. That second sprocket wheel is mounted on a shaft whose rotational axis is generally parallel to the rotational axis about which the drive sprocket wheel rotates. On the end of this shaft is the output member that then transfers power to the rear drive shaft of the vehicle.
As mentioned above, the power transfer unit of the present invention may, in a second embodiment, include a front wheel differential. Input into the front differential is through a ring gear coupled to rotate with the first bevel gear of the non-parallel gear set. Planet gears engage the ring gear and are supported by a carrier that in turn forms or is connected to one output of the front differential. The planet gears also engage a sun gear. The sun gear is mounted to the other differential output, one output being coupled to the left front wheel half-shaft with the other output being coupled to the right front wheel half-shaft. During normal straight line driving, these gears do not turn relative to each other. Rather, the entire planetary assembly rotates. During turning of the vehicle, these gears will rotate relative to one another. By incorporating a planetary style differential with the present invention, additional packaging space becomes available in order to provide adequate space for the helical gear set without adding to the overall transaxle space requirements.
By locating the non-parallel gear set so as to receive power from the transmission assembly, tighter packaging can be achieved in the power transfer unit, in particular along the longitudinal axis of the motor vehicle. By utilizing the non-parallel gear set before the parallel gear set, the parallel gear set can be positioned immediately adjacent, and as close as possible, to the transmission assembly thereby minimizing motion of the power transfer unit output via the close proximity to the powertrain roll axis. Transmitting power back to the centerline and at the desired height relative to the vehicle is readily accomplished through the parallel gear set as well. This further enables the offset in a hypoid bevel gear to be reduced thereby increasing the efficiency of the non-parallel gear set.
Additionally, the front wheel differential can be incorporated into the power transfer unit, increasing packaging flexibility. This readily allows for the conversion of front wheel drive vehicles into all wheel drive vehicles with a minimum impact on the other components packaged in the engine bay.
In one aspect, the present invention is therefore seen to be a power transfer unit comprising an input member adapted to connect to an output portion of a transmission assembly; an output member adapted to connect to an input portion of a rear wheel drive shaft; a non-parallel gear set coupled to receive power from the input member and deliver it to a parallel gear set; and the parallel gear set being coupled to deliver power to the output member.
In another aspect, the invention comprises the features recited in the previous paragraph and further a front wheel differential. The input of the front wheel differential is also directly coupled to the input member of the power transfer unit. The outputs of the front wheel differential provide the power to the front right and left half-shaft assemblies.
Additional objects and features of the present invention will be readily apparent to those skilled in the art from a review of the attached drawings, the following detailed description and the appended claims.